DESCRIPTION: The objective of this research is to obtain quantitatively accurate characterizations of the molecular organization of lipid molecules in bilayers. Such characterization is essential to test the hypothesis that the differences in lipid composition in different biomembranes in the body are related to structural differences in the underlying lipid bilayers that would be required for healthy cell function. The primary proposed technique is x-ray scattering which will be complemented by our volumetric measurements, as well as results from other laboratories using NMR, IR and molecular dynamics simulations. Electron density profiles of bilayers, composed primarily of phosphatidylcholine and phosphatidylethanolamine lipids with varying chain length and unsaturation, will be obtained at higher spatial resolution in the biologically relevant L-alpha phase. Bilayer preparations will include (i) multilamellar vesicles, (ii) oriented samples on solid substrates and (ill) unilamellar vesicles. Together with our determinations of gel phase structure, the area per lipid AF and the thickness of these bilayers will be determined. Thermal expansivity and the effects of temperature on membrane fluidity will be determined by measuring over wider temperature ranges. A necessary intermediate step will be to establish the dehydration threshold for structural invariance, so that less than fully hydrated and oriented samples with more orders of reflection can be studied, with the assurance that they have the same structure as fully hydrated bilayers. It is proposed to continue using the CHESS facility to obtain data at the necessary high instrumental resolution that can only be achieved for lipid bilayer samples with the high x-ray flux of synchrotron sources. These synchrotron studies will also yield information about the undulation fluctuations that play a role in interbilayer interactions. In addition, intrabilayer fluctuations that are necessary for functioning biomembranes will be studied by measuring wide angle diffuse scattering.